A tiltmeter is a device with a sensitivity sufficiently high to detect the lateral component when there is a deviation from the direction of the earth's gravitation vector. If an accelerometer is used the accelerometer senses the lateral component of the gravitation vector when the input axis of the accelerometer is deviated from the earth horizontal. A pendulum is a particular type of accelerometer in which the pendulum arm or length provides a convenient leverage for the suspension system as well as providing the elastic constraint for centering the mass.
In the case of the pendulum, sensitivity is increased by increasing the pendulum length. For instance if it is desired to measure the tilt of 10.sup.-.sup.4 radians with a displacement sensor placed near the pendulum mass and having a full scale displacement of 1 mm., then the length of the pendulum should be 10.sup.4 times 1 mm., or 10 meters.
For extremely sensitive applications, for example a tiltmeter sufficient to detect the earth tide corresponding to -- although very much smaller than -- the ocean tide, it has been necessary to provide pendulums in the order of 50 ft. long. Unfortunately, when pendulums become much longer than one tenth meter they are too bulky for portable application and also too susceptible to the varying effects of environment such as temperature changes.
The present invention solves these problems by providing a pendulum structure whose effective length is many times the actual physical length of any part of the structure. My tiltmeter involves the use of an ordinary pendulum coupled to an inverted pendulum. The coupling is arranged so that the effective length is many times the actual length. For example, I have found it possible to couple two 10 cm. pendulums together and provide a resultant system equivalent to a 50 meter long simple pendulum.
While jeweled bearings are normally considered to be extremely low friction devices, even the friction of a jeweled bearing could prevent the achievement of the sensitivities otherwise possible with tiltmeters according to my invention. I have devised a two-bar, two-strap or ribbon suspension system which gives extremely low pivotal friction while providing rigidity in all other axes. The swing of one bar with respect to the other involves the wrapping up of two sections of the ribbon while unwrapping two similar sections so that there is zero energy change involved.
Accordingly it is an object of my invention to provide an extremely sensitive tiltmeter having a physical size very much smaller than otherwise required by conventional pendulum structures. A further object is to provide a suspension system for the pendulum which provides extremely low rotational friction but high rigidity in all other axes.